1. Field of the Invention
This invention relates to semiconductor fabrication processes, and more particulary, to a method for determining the impurity concentration of impurity-doped polysilicon layers in a semiconductor wafer.
2. Description of Related Art
A semiconductor material, as the name implies, has electrical conductivity intermediate between that of a conductor and an insulator. One conventional method for increasing the conductivity of a semiconductor material, is to add an impurity material, called dopant, to the crystal structure of the semiconductor material to change the energy levels of its donors or acceptors. This forms a so-called extrinsic semiconductor whose electrical properties are dependent on the type and concentration of impurities added thereto. For instance, for silicon with a valence of 4, if an impurity material having a valence of 5, such as phosphorus, is added to the crystal structure of the silicon, the silicon will serve as an N-type semiconductor with increased conductivity due to an increased energy level caused by the presence of the impurity atoms.
By conventional inspection techniques, it is possible to ascertain whether an impurity-doped layer is adequately or inadequately doped with the impurity material because it can be checked by using a thermal probe, such as the Therma Probe 420 (TP420), to determine the current impurity concentration of the impurity-doped layer. In this method, a laser beam is produced by the thermal probe to illuminate the impurity-doped layer, and then the amount of thermal wave (TW) resulting from the illumination of the laser beam in the impurity-doped layer is measured. The detected TW value can be used to determine the impurity concentration of the impurity-doped layer.
The foregoing method, however, is only suitable for use on single-crystal silicon and not suitable for use on polysilicon. In an experiment, for example, five sample wafers formed with impurity-doped polysilicon layers are prepared. Then, the TW values of the impurity-doped polysilicon layers in these sample wafers are measured. The results are shown in Table 1.
From the data shown in Table 1, it is apparent that the TW values of the impurity-doped polysilicon layers display no regularity or repetitiveness. Consequently, it would be almost impossible to determine whether the impurity-doped polysilicon layers are adequately or still inadequately doped with the impurity material by checking on the TW values thereof. It is also impossible to determine the current impurity concentration of the impurity-doped polysilicon layers. Therefore, for an inadequately doped polysilicon layer, the compensation thereof can be carried out only by relying on an implantation data log (IMP. Data Log) recorded by the implantation machine, or on manually recorded data by the operating personnel. This practice is, of course, quite inaccurate. In the event of shutdown of the machine, software bugs, or failure to record data by the operating personnel, the whole wafer should be discarded since the impurity concentration of the impurity-doped polysilicon layers thereon is unknown.
Even if detailed and accurate records are available, the wafers that are formed with adequately doped polysilicon layers can be disorderly arranged due to malfunction of the sorter (a machine for sorting the wafers) or human error. This will preclude checking the wafers based on their TW values, resulting still in the necessity of discarding the wafers.